Recently, in rapidly growing mobile communication systems (for example, Personal Handyphone System, abbreviated as PHS hereinafter), a PDMA (Path Division Multiple Access) scheme is proposed where path division multiple access is established between radio terminal devices (terminals) of a plurality of users and a radio base station (base station) by spatially dividing the same time slot at the same frequency in order to increase the frequency utilization efficiency of radio waves.
In this PDMA scheme, the adaptive array technique is employed at present where an up-link signal from an antenna of each user terminal is received by an array antenna at a base station and separated and extracted with reception directivity through adaptive array processing. On the other hand, a down-link signal from the base station to the terminal is transmitted from the array antenna with transmission directivity to the antenna of the terminal.
Such adaptive array processing is a well-known technique and described in detail, for example, in “Adaptive Signal Processing by Array Antenna” by Nobuyoshi Kikuma (Kagaku Gijutsu Shuppan,) chapter 3, MMSE Adaptive Array, pp 35 to 49, and therefore the description of the operation principle will not be made here.
It is noted that in the following description, a base station performing down-link transmission directivity control for a terminal using such adaptive array processing will be referred to as an adaptive array base station.
On the other hand, such a terminal is known that performs selective diversity reception (diversity reception, hereinafter) using a plurality of antennas. As such a terminal, a terminal including two antennas called a whip antenna and a chip antenna is common, which will be referred to as a diversity terminal hereinafter. Furthermore, a terminal with one antenna that does not perform diversity reception will be referred to as a normal terminal.
This diversity terminal normally operates to fix one antenna (generally a whip antenna) as a transmission antenna at the time of transmission and to select one of the whip antenna and the chip antenna which has a higher reception level as a reception antenna at the time of reception.
Such a conventional diversity terminal carries out the aforementioned diversity reception irrespective of whether a base station of the other party for access is an adaptive array base station performing transmission directivity control or a non-directional base station.
FIG. 9 is a diagram schematically showing an exemplary connected state between a terminal and a base station, and more particularly a diagram schematically showing path division multiple access from a diversity terminal and a normal terminal to an adaptive array base station through adaptive array processing.
Referring to FIG. 9, a diversity terminal 2 transmits an up-link signal with a whip antenna 2a fixed as a transmission antenna at the time of transmission, and an adaptive array base station 1 receives this signal with an array antenna 1a. 
On the other hand, a normal terminal 3 transmits an up-link signal with one antenna 3a at the time of transmission, and adaptive array base station 1 receives this signal with array antenna 1a. 
In response, adaptive array base station 1 transmits a down-link signal to diversity terminal 2 with directivity (which corresponds to a transmitting power level) indicated by the solid line, and transmits a down-link signal to normal terminal 3 with directivity (which corresponds to a transmitting power level) indicated by the broken line.
The transmission directivity for diversity terminal 2 is such that a beam is directed to whip antenna 2a of diversity terminal 2 that has transmitted the up-link signal and null is directed to antenna 3a of normal terminal 3 that is an interference source.
On the other hand, the transmission directivity for normal terminal 3 is such that a beam is directed to antenna 3a of normal terminal 3 that has transmitted the up-link signal and null is directed to whip antenna 2a of diversity terminal 2 that is an interference source.
As a result, the signal received at antenna 3a of normal terminal 3 is a signal with a small interference component, that is, with a so-called DU (Desired user's power: Undesired user's power) ratio being high, where the down-link signal level (the solid line) from adaptive array base station 1 to diversity terminal 2 is relatively small with respect to the down-link signal level (the broken line) from adaptive array base station 1 to normal terminal 3.
The signal received at whip antenna 2a of diversity terminal 2 is also a signal with a high DU ratio where the down-link signal level (the broken line) from adaptive array base station 1 to normal terminal 3 is relatively small with respect to the down-link signal level (the solid line) from adaptive array base station 1 to diversity terminal 2.
As described above, the diversity terminal operates to fix the whip antenna as a transmission antenna at the time of transmission and to select one of the whip antenna and the chip antenna which has a higher reception level as a reception antenna at the time of reception.
Here, with reference to chip antenna 2b of diversity terminal 2, this antenna is not used to transmit the up-link signal, and the null of the transmission directivity (the broken line) for normal terminal 3 is not directed.
Therefore chip antenna 2b of diversity terminal 2 receives the transmitting power (the broken line) for normal terminal 3 in addition to the transmitting power (the solid line) for diversity terminal 2. As a result, there is a possibility that the reception level in chip antenna 2b may exceed the reception level at whip antenna 2a, resulting in that chip antenna 2b may be selected as a receiving antenna.
If chip antenna 2b is selected as a receiving antenna, the down-link signal level (the broken line) from adaptive array base station 1 to normal terminal 3 becomes relatively large with respect to the down-link signal level (the solid line) from adaptive array base station 1 to diversity terminal 2, resulting in a signal with a large interference component and a low DU ratio.
Even if such a received signal with a low DU ratio is demodulated in diversity terminal 2, the frame of the demodulated signal will suffer from an error to prevent accurate demodulation.
As described above, in the conventional mobile communication system where a plurality of terminals including a diversity terminal establish access to an adaptive array base station, a down-link received signal at the diversity terminal has a reduced DU ratio and the reception performance is deteriorated because of interference wave. As a result, the diversity terminal will not be able to maintain path division multiple access.
An object of the present invention is therefore to provide a radio base apparatus, a transmitting power control method and a transmitting power control program, free from deteriorated reception performance in a diversity terminal, in a mobile communication system where a plurality of terminals including a diversity terminal establish access to an adaptive array base station.